U.S. patent number 3,972,624 [Application Number 05/531,682] was granted by the patent office on 1976-08-03 for process and apparatus for detecting longitudinal faults on moving webs of material.
This patent grant is currently assigned to AGFA-Gevaert, A.G.. Invention is credited to Julius Geiger, Hans Joachim Klein, Manfred Rupprecht, Heinz Wonneberg.
United States Patent |
3,972,624 |
Klein , et al. |
August 3, 1976 |
Process and apparatus for detecting longitudinal faults on moving
webs of material
Abstract
In a process for detecting longitudinally orientated faults on
moving webs of paper or film, the web is optically scanned line by
line transversely to its direction of movement and read by
reflection. The scanning line is moved to and fro across the whole
web at constant velocity and during a given number of scanning
movements the instantaneous value of the reflected signal is
interrogated at two fixed time marks within the scanning line and
stored and the average values over a scanning cycle are formed
separately for each of the two time marks. The average values ae
indicated and recorded as faults only if a value different from the
average noise value is obtained from both time marks in
succession.
Inventors: |
Klein; Hans Joachim (Wuppertal,
DT), Rupprecht; Manfred (Leverkusen, DT),
Wonneberg; Heinz (Leverkusen, DT), Geiger; Julius
(Odenthal-Gloebusch, DT) |
Assignee: |
AGFA-Gevaert, A.G. (Leverkusen,
DT)
|
Family
ID: |
5901344 |
Appl.
No.: |
05/531,682 |
Filed: |
December 11, 1974 |
Foreign Application Priority Data
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|
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Dec 20, 1973 [DT] |
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2363422 |
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Current U.S.
Class: |
356/431;
356/237.1; 139/1B; 250/559.49; 250/559.06; 139/113 |
Current CPC
Class: |
G01N
21/8921 (20130101) |
Current International
Class: |
G01N
21/88 (20060101); G01N 21/892 (20060101); G01N
021/32 () |
Field of
Search: |
;356/199,200,202,237,238
;250/562,563,572 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Corbin; John K.
Assistant Examiner: Rosenberger; Richard A.
Attorney, Agent or Firm: Connolly and Hutz
Claims
What we claim is:
1. A process for detecting and localising longitudinally orientated
faults on moving webs, in which the web is scanned optically line
by line trasversely to its direction of movement and read by
reflection, wherein:
a. the scanning line is reciprocated transversely over the whole
web at a constant velocity,
b. the instantaneous value of the reflected signal is interrogated
periodically at two fixed time marks within the scanning line
during a predetermined number of scannings which make up a scanning
cycle and stored and the values obtained in the course of a
scanning cycle are averaged separately for both time marks, and
c. the average values are indicated and processed as faults only if
a value deviating from the average noise value is supplied from
both time marks.
2. A process according to claim 1, wherein the instantaneous values
interrogated at the two time marks are stored as analogue values
and then converted into digital form and the average value for each
time mark is then formed digitally from these values.
3. A process according to claim 1, wherein the two time marks
within the scanning line are determined by means of a line starting
pulse.
4. A process according to claim 1, wherein the output of a source
of light supplying the light for the optical scanning is so
controlled that the intensity of the beam reflected from the
surface of the web remains constant.
5. A process according to claim 4, wherein the incoming pulse or
outgoing pulse appearing at the beginning or end of each scanning
line is used to effect the said control.
6. An optical reflection arrangement for detecting and localising
longitudinally oriented faults on moving webs, comprising a source
of light, a mirror wheel and, focusing device (together
constituting a scanner) and a photo-electric detector in an
automatic collimation arrangement, and an electronic processing
circuit for processing signals delivered by the detector, wherein
the scanner and detector are mounted on a carriage which is
displaceable transversely to the direction of movement of the web
so that the scanning line of the scanner traverses the moving web,
and the electronic processing device comprises a circuit which
carries out a quasi-continuous formation of the average value of
the signals delivered by the detector.
7. An arrangement according to claim 6 wherein the digital control
circuit comprises two monostable multivibrators with which an AND
gate and an OR gate are connected in series, and two monostable
multivibrators are connected into the circuit in such a way that
they are unambiguously associated with the two directions of
traversing so that at any given time only one of the last mentioned
multivibrators is ready for operation, depending on the direction
of traversing.
8. An arrangement according to claim 6, wherein the electronic
processing device comprises:
a. a sample and hold member adapted for both positive and negative
voltages, which is switched in the rhythm of the time marks to
"hold" for a length of time which is shorter than the time between
the scanning marks and to "sample" for the rest of the time;
b. two voltage frequency converters one for positive voltages and
the other for negative voltages, each of which converters converts
the voltage values at the output of the sample and hold member into
a pulse sequence with a frequency proportional to the voltage and
transmits the hold values to two forward-backward counters
electrically connected to the time marks during a conversion period
which lies within the holding time, in which counters the digital
formation of the average value takes place and at the outputs of
which a pulse is produced if, starting from a preset value, an
upper limit is exceeded in the case of forward counting or the
average value falls below a lower limit in the case of backward
counting, and
c. a digital control circuit which is controlled synchronously with
the traversing device and in which the pulses appearing at the
outputs of the forward and backward counters are transmitted to the
recording instrument only if a pulse is delivered successively from
each of the two forward-backward counters and the time sequence of
these pulses corresponds to the sequence with which the two time
marks successively cover a fault signal produced by one and the
same fault in the course of a traverse movement.
9. An arrangement according to claim 8, wherein a line starting
pulse is produced by a photoelectric diode and then amplified and
standardised and then used to trigger a first monostable
multivibrator which has second, third and fourth monostable
multivibrators successively connected in series therewith the
output of the first multivibrator and the output of the third
multivibrator being connected to a control input of the sample and
hold member by way of an OR gate, and at the changeover from the
sample function to the hold function, one time mark being fixed by
the first multivibrator and the following time mark being fixed by
the third multivibrator.
10. An arrangement according to claim 9, wherein the conversion
time during which the voltage pulses produced by the voltage
frequency converters are counted by the forward-backward counters
is fixed by the second and fourth monostable multivibrators the
second multivibrator producing the conversion time belonging to one
time mark and the fourth multivibrator producing the conversion
time belonging to the following time mark.
11. An arrangement according to claim 9, wherein the standardised
time starting pulse energises a reducer in which the scanning cycle
is fixed and after each scanning cycle both forward-backward
counters are returned to the preset starting value by the output
signal of the reducer, which signal is converted in a monostable
multivibrator.
12. An arrangement according to claim 9, wherein the two voltage
frequency converters are associated with the two time marks by
means of gates and the monostable multivibrators.
13. An arrangement according to claim 12, wherein the two of the
gates are switched off at a respective input when that time mark
which is closest to the edge of the web in the traversing direction
leaves the surface of the web while the following time mark is
still on the surface of the web.
Description
The invention relates to an optical process of and an apparatus for
detecting, and recognising and localising longitudinally orientated
faults on moving webs of paper or film, in particular on
photographic materials. The web is scanned line by line
transversely to its direction of movement and read by
reflection.
In the manufacture of webs of material, in particular coated films
such as photographic films and paper, faults may occur in the web,
and particularly in the layers applied to the support, which may
seriously impair the subsequent use of the web. Although punctiform
faults such as specks of dirt, bubbles, etc. may occur it is faults
extending in the longitudinal direction of the web, such as casting
stripes or scratches, which may frequently extend over the whole
length of the web, which are particularly likely substantially to
reduce the quality. It is absolutely essential to detect such
faults which extend longitudinally of the web.
Methods of detecting longitudinal faults are known in which the web
is scanned optically by reflected or transmitted light of a
suitable wavelength. This can be done, for example, by illuminating
the moving web in uniform strips transversely to the longitudinal
direction of the web and transmitting the reflected light (U.S.
Pat. No. 3,206,606), or the transmitted light in the case of
optically transparent webs (U.S. Pat. No. 3,286,567), to a
photoelectric receiver or to a plurality of such receivers arranged
side by side and parallel to the illuminated strip of web. To
detect faults extending parallel to the longitudinal direction of
the web, the detector arrangement may be reciprocated transversely
to the longitudinal direction of the web. Intensity differences due
to faults are perceived as light pulses by the photoelectric
receivers and transmitted from the receivers to instruments for
recording, controlling or storing the pulses.
In another process, a perforated band is moved transversely to the
longitudinal direction of movement of the web and light is passed
through the band so that the web is scanned in bands by the points
of light produced by the perforations. The points of light passing
through the web are then transmitted on the other side of the web
to a plurality of photoelectric receivers or to a single detector
by way of a photo-conductive rod (U.S. Pat. No. 3,331,963). Other
scanning devices are known in which a patch of light is moved line
by line transversely over the moving web by means of a rotating
mirror, mirror wheel or similar optical element and the reflected
light (e.g. U.S. Pat. Nos. 3,646,353 and 3,510,664) or the
transmitted light in the case of optically transparent webs (U.S.
Pat. No. 3,556,664) is passed through a lens system to be projected
on to at least one photoelectric receiver.
In another process for detecting faults which extend parallel to
the longitudinal direction of the web, the surface of the web is
scanned by means of two fixed patches of light which are at a
constant distance apart and staggered in line so that the light
reflected from the web is transmitted to a separate photoelectric
receiver for each of the two light patches and the whole scanning
head is reciprocated across the width of the web transversely to
the longitudinal direction of the web (German Offenlegungsschrift
No. 1,573,801).
In another process which is employed especially for detecting
longitudinal stripes (casting stripes) on photographic material in
the moist state, the web is scanned by a reciprocating patch of
light at the infrared absorption bond of water specifically to
detect water (U.S. Pat. No. 3,564,265).
The methods described above for detecting faults on moving webs of
material have one particular disadvantage. The light reflected or
transmitted from the web and received by the photo-detector is
modulated by the properties of the material of the web even where
there are no faults since even a fault-free web is not normally
completely uniform. The signal produced in the photo detector
therefore has a certain irregularity (background noise) even in the
absence of faults. In most cases where the methods described above
are employed, the background noise is cut off electronically by an
amplitude cut-off device such as a Schmitt-Trigger or a limiter
diode, so that faults can be detected on the web of material only
if the electrical pulse they produce is greater than the maximum
interfering pulses and therefore greater than the discriminator
voltage. Some improvement is obtained by a method in which a
modified output signal of the photo detector is formed which is
characteristic of the peak noise averaged over a time, and this
output signal is compared with the original output signal of the
photo-detector by means of a voltage comparator so that unwanted
noise components are suppressed and the comparator yields an output
signal only in the presence of a genuine fault pulse. In order to
reduce the mistaken indications even further in faulty material the
output signal of the comparator can in addition be transmitted to a
discriminator with a constant threshold voltage (U.S. Pat. No.
3,510,664). Additional frequency-determining methods in the form of
filters to separate pulses due to faults in the material from
background noises (e.g. U.S. Pat. Nos. 3,510,664 and 3,206,606) do
not provide a good solution since in practice there is usually
hardly any difference in the frequency between pulses produced by
faults and background noise. These arrangements serve merely to
filter out unwanted interference frequencies which do not fall
within the range of measured frequencies.
The detection of small faults, i.e. faults which give rise to
pulses with an amplitude which is the same as or smaller than the
noise amplitude cannot be achieved with the known methods mentioned
above, but it is precisely the detection of such fine faults which
is particularly important for the quality control of material webs,
in particular of photographic material, if they are in the form of
longitudinal faults often extending over whole length of the
web.
It is an object of this invention to improve the optical detection
of faults on moving webs of material to such an extent that even
very fine longitudinal faults extending parallel to the direction
of movement of the web of material, such as scratches or stripes,
can be found with certainty. In particular, it is intended that
such longitudinal faults on photographic materials should already
be detected and recorded during the course of production. The fault
detector arrangement according to the invention must therefore be
so constructed that it can be installed directly behind a coating
apparatus and that it can detect longitudinal faults while the web
of material is still wet. Since point faults are often less
significant in spoiling the quality of photographic materials than
longitudinal faults, they should be eliminated by the
instrument.
According to the invention, the problem is solved by means of an
optical reflection scanner which scans the moving web line by line,
the length of the scanning line being substantially smaller than
the width of the web of material. The light which is reflected by
the web and modulated by the surface properties of the web is
transmitted to a photoelectric receiver fixed to the scanner. The
process according to the invention is characterised in that
a. the scanning line is reciprocated over the whole width of the
web at a constant velocity,
b. the instantaneous value of the reflected signal is interrogated
periodically on two fixed time marks within the scanning line in
the course of a given number of scanning operations (scanning
cycle) and stored and the average values for each of the two time
marks over a scanning cycle are formed separately, and
c. the average values are indicated and recorded as faults only if
a value different from the average noise value is supplied from
both time marks in succession.
The instantaneous values interrogated at the two time marks are
preferably stored as analogue values and then digitalised and the
average value formed digitially for each time mark.
After expiry of such an operation for obtaining the average value
during the given number of scanning movements, the average value
formation begins again from the beginning for the next scanning
cycle. While the scanning head traverses the width of the web, the
two scanning marks also shift in the corresponding traversing
direction. Where the web is free from faults, the statistically
distributed background noise of the web which is due to the
characteristic surface properties of the web is averaged out by the
process while a longitudinal fault, even if fine, which produces a
uniform positive or negative fault pulse during the scanning
operation, will not fall within the average noise level but will
produce a different value and will therefore be detected. In order
to be recognised, the fault pulse must be detected successively in
the correct sequence by each of the two time marks (scanning marks)
which shift in the given traversing direction during the traverse
motion. If the longitudinal fault is detected by the first time
mark (scanning mark), the information of this fault is stored until
the second time mark (scanning mark) has also passed across the
fault. If the fault is recognised also by this second mark, then
the information of detection of a fault is available for both time
marks (scanning marks) simultaneously and the fault is recorded as
a longitudinal fault. The minimum length of a fault which will
still be detected as longitudinal fault therefore depends on the
velocity of the traversing movement and the velocity of transport
of the web of material. The lower the velocity of the web and the
higher the velocity of traversing, the smaller will be the minimum
length of a fault which will still be detected as a longitudinal
fault. Faults in the form of points, which are often insignificant,
are not detected by the process because the probability of the
first time mark (scanning mark) falling on a point fault and the
second time mark (scanning mark) also passing over a second point
fault after expiry of the storage time is very slight.
The two time marks within the scanning line are preferably set by a
line starting pulse.
In a preferred embodiment of the invention, the electrical output
of the source of the beam (source of light) is regulated so that
the intensity of the beam reflected from the surface of the web
remains constant. The incoming or outgoing pulse appearing at the
beginning or end of each scanning line is used to control this
regulation.
The optical reflection system for carrying out the process
according to the invention consists of a source of light, a mirror
wheel with a focusing device (scanner) and a photoelectric detector
arranged as an automatic collimating system, and an electronic
processing circuit to process the signals supplied from the
detector. The characteristic feature of this arrangement is that
the scanner including the detector is mounted on a carriage which
is movable transversely to the direction of the web so that the
scanning line of the scanner traverses the moving web, and the
electronic processing apparatus consists of a circuit which carries
out quasi-continuous averaging of the signals from the
detector.
The electronic processing circuit advantageously consists of the
following units:
a. a sample and hold member suitable for positive and negative
voltages which is switched to "hold" in the rhythm of the time
marks for a period which is shorter than the time between two
scanning marks and remains switched to "sample" for the rest of the
time;
b. a voltage-frequency converter both for positive and for negative
voltages, which converts the voltages at the output of the sample
and hold member into a pulse sequence with a frequency proportional
to the voltage and transmits the hold values during a conversion
time lying within the hold time to two forwards-backwards counters
connected into the circuit of the two time marks, in which counters
the digital formation of the average value takes place and at the
outputs of which counters a pulse is produced when, starting from a
pre-adjusted value, an upper limit is exceeded in the case of
forward counting or the value falls below the lower limit in the
case of backward counting;
c. a digital control circuit controlled synchronously with the
traversing device, in which the pulses at the output ends of the
forward-backward counters are transmitted to the registering
instrument only when a pulse is supplied successively from each of
the two forward-backward counters, and only if the time sequence of
these pulses corresponds to the sequence with which the two time
marks successively detect a fault signal produced by one and the
same fault in the course of their traversing movement.
The line starting pulse to set the two time marks is advantageously
produced by means of a photoelectric diode installed in the
scanner. The line starting pulse is subsequently amplified and
standardised and transmitted to a monostable multivibrator which is
triggered by it. Three other monostable multivibrators aare
connected in series with this first multivibrator. The output of
the first multivibrator and the output of the third multivibrator
are connected to the control input of the sample and hold member by
an OR gate. The effect of this circuit is that on transition from
sample to hold function, one time mark is fixed by the first
multivibrator and the following time mark by the third
multivibrator. The voltage pulses by the two voltage frequency
converters are transmitted to the next following forward-backward
counters during a certain conversion time. The conversion time
during which the voltage pulses produced by the voltage frequency
converters are counted by the forward-backward counters is
determined by the above mentioned multivibrators connected in
series and an additional monostable multivibrator (fourth
multi-vibrator). The second multivibrator produces the conversion
time T.sub.2 belong to one time mark and the fourth multivibrator
produces the conversion time T.sub.4 belonging to the following
time mark.
The scanning cycle is preferably determined by the line starting
pulse. For this purpose, the standardised line starting pulse
controls a reducer in which the scanning cycle is fixed. In
addition, after each scanning cycle, both forward-backward counters
are reset to their preadjusted starting value by the previously
converted output signal of the reducer.
The digital control circuit at the output end of the electronic
processing circuit consists of two monostable multi-vibrators
followed in series by an AND gate and an OR gate. The last two
mentioned monostable multivibrators are unambiguously connected
with the two traversing directions of the scanner so that there is
always only one multivibrator ready in operation for the
appropriate direction of traversing.
The voltage frequency converter is also preferably connected to the
two time marks by the four gates and monostable multivibrators
which are triggered by the line starting pulse.
In this arrangement, two gates are always switched off alternately
when the time mark closest to the edge of the web in the direction
of traversing leaves the surface of the web while the next
following time mark still covers the surface of the web.
The advantages of the invention lie in the high degree of
sensitivity with which faults in the form of stripes can be
detected. The sensitivity is so high that even stripes which
produce a reflection signal far below the noise level can be
reliably detected. In addition, stripes can be clearly
distinguished from point faults. The method used in the invention
of forming the average value also ensures that the circuit is
largely independent of external disturbances. Another major
advantage is that the same apparatus can also be used for finding
point faults. The reflection signals interrogated at the two time
marks are then averaged in the same way but not logically linked
together at the output of the electronic processing circuit as
described above. Instead, the average values or only one of the two
average values are registered directly. Even fast moving webs (2
meters per second) can be scanned without any gaps.
The structure and mode of operation of the invention will now be
described in more detail with reference to an embodiment shown in
the drawings.
FIG. 1 shows schematically the arrangement of the optical
reflection scanner,
FIG. 2 shows the signal for a line scanning received by the
photoelectric receiver and the corresponding line starting
pulse,
FIG. 3 is a schematic view of the traversing arrangement,
FIG. 4a a block circuit diagram of the electronic processing
circuit,
FIG. 4b is a pulse-time diagram corresponding to FIG. 4a for two
successive line scannings of the optical scanner,
FIG. 4c shows schematically the scanning of a longitudinal fault
pulse at various times during the traversing movement,
FIG. 5 shows schematically the recognition of longitudinal faults
at the edge of the web,
FIG. 6a is a block circuit diagram of the means for controlling the
intensity of the light source,
FIG. 6b is the pulse-time diagram corresponding to FIG. 6a.
FIG. 1 is a simplified view in perspective of an embodiment of an
optical scanner. The scanner includes a source of light 1 which is
a tungsten lamp. The light passes through a semi-transparent mirror
2 arranged at an angle of, for example, 45.degree. to the incident
light and through an inlet gap 3 which is adjustable in height and
width and which is fixed to the back of the mirror 2.
A rotatable mirror wheel 5 covered with plane mirrors 4 is rotated
by a motor 6 so that as the wheel rotates the mirrors 4 move in the
focus of a parabolic mirro 7 which is arranged parallel to the web
of material 8 which itself moves in the direction indicated by the
arrow. To avoid irregularities in the movement of the web 8, the
web passes over two rollers 9 and 10 which are placed close
together and over which the web is wrapped with as large a looping
angle as possible.
A convex lens 11 focuses the light from the gap 3 on to the mirrors
4 of the mirror wheel 5 which reflect the light on to the parabolic
mirror surface 7 which in turn projects it as a small scanning spot
3a on to the web of material 8 with a direction of incidence
perpendicular to the surface of the web. Since the mirror wheel is
situated below the parabolic mirror 7, the wheel 5 and mirror 7 are
set at a slight angle to each other for purposes of reflection and
focusing. The light reflected by the web 8 and modulated by the
surface properties of the web retraces its path to return to the
semi-transparent mirror 2 by way of the parabolic mirror 7, mirrors
4 and lens 11. It is then deflected by the mirror 2 and focused by
a convex lens 12 on to a photoelectric receiver 13 which may, for
example, be a rapid, highly sensitive and as far as possible
low-noise photoelectric diode or photo-transistor. The scanner thus
operates on the principle of automatic collimation.
When the mirror wheel 5 rotates in the sense indicated, the beam
reflected by the parabolic mirror 7 shifts parallel to itself in
the direction indicated so that the scanning spot 3a scans the web
8 along the scanning line 14. The system is so arranged that only
one mirror 4 of the mirror wheel 5 is used for each scanning
operation. If the web 8 moves in the direction of the arrow, it is
therefore scanned line by line without gaps along the length of the
scanning line 14.
A fault on the web of material is detected by the scanning spot 3a
and transmitted via the parabolic mirror 7, mirror wheel 5, lens
11, semi-transparent deflecting mirror 2 and lens 12 to the
photoelectric detector 13 which converts it into an electrical
fault pulse which is then processed in an electronic circuit, which
will be described with reference to FIG. 4, where it is recognised
and recorded.
When the system is used to check photographic material, an infrared
filter 15 which absorbs light in the visible spectrum is arranged
between the parabolic mirror 7 and web 8.
The entrance and exit of the scanning means at the ends of the
parabolic mirror 7 produce pulses but these do not interfere with
the process. On the contrary, they can be used to control the
intensity automatically, as will be explained in more detail
below.
Laterally to the parabolic mirror, another photoelectric diode 16
is arranged in the path of the beam of light so that it is swept by
the beam and produces a line starting pulse before the beginning of
each scanning operation. Other means could be used for producing
this line starting pulse, e.g. inductive scanning of the mirror
wheel.
By way of example consider the case where the scanning width is 7
cm, equal to the length of the scanning line 14, and the speed of
rotation of the mirror wheel 5 is 50 revs. per sec. If the number
of mirrors 4 is 16, the scanning frequency is 800 Hz and hence the
whole operational time of one mirror 4, that is to say the sum of
the actual scanning time T.sub.a on the web 8 and a certain dark
time T.sub.d between two mirrors, which depends on the geometry and
number of mirrors, is T = 1.25 ms. If the rate of movement of the
web is 40 m/min, for example, the web 8 during this time (1.25 ms)
moves forward by 0.83 mm. Since the length of the scanning spot 3a
in this example is 2 mm, the line by line scanning of the web
leaves no areas thereof unscanned. If the web moves at higher
speeds, complete scanning without gaps can still be achieved, e.g.
by increasing the speed of rotation of the mirror wheel 5 or by
increasing the length of the scanning spot 3a.
FIG. 2a shows the signal received by the photoelectric receiver 13
when a line of fault-free web is scanned. The signal is then
modulated only by the normal surface properties of the web. The web
is scanned from left to right. 17 is therefore the entrance impulse
which is dependent upon the surface of the web and 18 is the
associated exit impulse. T = 1.25 ms is the total operational time
of the mirror, which is composed of the scanning time T.sub.a and
dark time T.sub.d.
FIG. 2b shows the associated line starting pulse 19 situated before
each line scanning and received by the photoelectric diode 16.
FIG. 3 is a simplified view in perspective of an example of a
traversing arrangement. 20 represents the optical scanner shown in
FIG. 1. The scanner 20 is fixed to a carriage 21 which is adapted
to move backwards and forwards on rails 23 and 24 parallel to the
surface of the web 8, the rails 23 and 24 being connected to a
stable support 22. As the carriage moves backwards and forwards
perpendicularly to the longitudinal direction of the web, the
scanning line 14 is moved backwards and forwards across the width
of the web in the directions indicated by the arrows and at the
same time the scanner 20 scans the web 8 along the scanning line
14. The drive for the traverse motion is provided by a motor 25,
drive sprockets 28, 29, 30 and 31 mounted in bearing blocks 26 and
27, and a sprocket belt 33 which moves over these sprockets and is
fixed to the scanner 20 by the clamp 32. When the carriage 21
reaches an end position, it is automatically switched to move in
the opposite direction by means of a motor control 34 which will
not be described in detail here and two limit switches 35 and 36
which are fixed to the bearing blocks 26 and 27 and adjusted to the
width of the web. The switches 35, 36 may, for example, be
inductive approximation initiators. The motor control 34 contains a
device which taps the voltage supply of the motor 25 and transmits
a signal in the form of a direct voltage dependent upon the given
direction of the traverse motion to the electronic processing unit
37 the operation of which will be described in more detail with
reference to FIGS. 4a and 4b. To fix the position of the scanner 20
in relation to the width of the web during the traverse motion, a
transmitter 38 is provided, e.g. in the form of a potentiometer or
a digital rotation transmitter, which is connected to the sprocket
30 and the signal of which is used in the electronic circuit 37 to
determine the coordinate on the width of the web of a longitudinal
fault.
FIG. 4a shows the circuit diagram of the electronic processing
circuit 37 (FIG. 3) and FIG. 4b is the associated pulse-time
diagram for two successive line scannings of the scanner (FIGS. 1
and 2). To explain the operation of the electronic circuit more
clearly, FIGS. 4a and 4b will be considered together. The signal
produced by the photoelectric receiver 13 in response to the
reflected beam of light which is modulated by the properties of the
surface of the web is amplified in the amplifier 39 to form a wide
band and enters the input of a sample and hold member 40 (e.g. type
SHA 1A of Analogue Devices, USA) which is suitable both for
positive and for negative input amplitudes. The signal obtained at
the output end of the amplifier 39 is shown in FIG. 4b (a) for two
successive scanning lines. 17 is the entrance pulse already
mentioned above and 18 the exit pulse for the first scanning line,
and 17' and 18' are the corresponding pulses for the following
scanning. T = 1.25 ms is the total operational time of one
mirror.
The line starting pulse produced by the photoelectric diode 16
before the beginning of each scanning line is amplified in the
amplifier 41 and transformed into a positive rectangular pulse in
the Schmitt Trigger 42. FIG. 4b shows the variation with time of
the line starting impulse 19 or 19' for the following scanning,
FIG. 4b (c) shows the rectangular pulse standardised from this line
starting impulse at the output of the Schmitt-trigger 42. After the
Schmitt-Trigger 42, four monostable multivibrators 43, 44, 45 and
46 are arranged in series. These multivibrators produce positive
rectangular pulses, and starting from the standardised line
starting pulse (FIG. 4b-c) they are triggered in each case by the
negative flank of the preceding pulse. FIG. 4b, (d) - (g) show the
variation of the time. FIG. 4b (d) represents the variation with
time of the monostable multivibrator 43, FIG. 4b (e) represents
that of the multivibrator 44, FIG. 4b (f) that of the multivibrator
45 and FIG. 4b (g) that of the multivibrator 46. The corresponding
pulse times are as follows: T.sub.1 = 160 .mu.s, T.sub.2 = 500
.mu.s, T.sub.3 = 40 .mu.s and T.sub.4 = 500 .mu.s. T.sub.2 should
be equal to T.sub.4. Two time marks for scanning, indicated by the
reference numerals 47 and 48 or 47' and 48', indicated by the
arrows, are determined by the negative flank (from L to O) of the
pulse produced by the multivibrator 43 (FIG. 4b (d)) and by the
negative flank of the pulse produced by the multivibrator 45 (FIG.
4b (f)). These time marks are situated approximately symmetrically
in relation to the pulses 17 and 18 or 17' and 18'.
The outputs of the monostable multivibrators 43 and 45 are passed
through the OR gate 49 to be brought together at the control input
of the sample and hold member 40. The variation with time of the
output of the OR gate 49 is calculated by adding the variations
with time in FIGS. 4b (d) and 4b (f). The result is shown in FIG.
4b (h).
If the control input of the sample and hold member 40 is at L
potential, that is to say during the times T.sub.1 and T.sub.3,
then the function sample is switched on. If on the other hand it is
at the O-potential, then the function hold is switched on. The
circuit is therefore always switched from sample to hold at the
time marks 47, 48 and 47', 48' and to sample at the O-L
transitions. The function sample means that the input signal is
always at the output of the sample and hold member while when the
circuit is switched to hold the signal value which is obtained at
the input at that moment is maintained as a fixed voltage value at
the output during the hold time. The output of the sample and hold
member 40 is connected to two voltage frequency converters 50 and
51, the converter 50 being suitable for positive input signals and
the converter 51 being suitable for negative input signals. Their
function is to convert the positive or negative value at the output
of the sample and hold member 40 during an exactly specified
conversion time within the hold time (of the sample and hold member
40) into a pulse sequence of specified rectangular pulses with a
frequency proportional to the said value. The conversion ratio is
10 kHz/V (e.g. Type W10 - 10B, BPS Electrical GmbH, Hanover).
As will be described in more detail below, the conversion time for
the instantaneous value fixed at the scanning mark 47 or 47' and
stored in the sample and hold member 40 is T.sub.2 = 500 .mu.s, and
for the scanning mark 48 or 48' it is also T.sub.4 = 500 .mu.s. The
conversion times for the two scanning marks are therefore
equal.
The output of the voltage frequency converter 50 for positive input
values is connected to one input of each of the AND gates 52 and
53, and the voltage frequency converter 51 for negative input
values is connected to one input of each of the AND gates 54 and
55. The output of the monostable multivibrator 44 is connected to
the second input of each of the gates 52 and 54 so that these gates
are energised only during the conversion time T.sub.2. The gates 52
and 54 are thus associated with the left hand scanning mark 47 or
47'. Their outputs are connected to the inputs of the
forward-backward counter 56 in such a way that the output of the
gate 52 is connected to the forward input 57 and the output of the
gate 54 is connected to the backward input 58 of the counter 56.
The forward-backward counter 56 can be preadjusted to a selectable
value, e.g. the number 32. Two outputs of the counter are also
preselected, the output 59 e.g. with the number 64 and the output
60 with the number 0. This means that when pulses reach the input
57, they are counted "forwards" from the number 32 onwards. When
the preselected number 64 is reached, a pulse appears at the output
59. This happens after exactly 32 input pulses. If, on the other
hand, pulses reach the backward input 58, they are counted
"backwards" from the preset number of 32 onwards until the number O
is reached, again after 32 pulses, and a pulse appears at the
output 60. The outputs 59 and 60 of the counter 56 are connected
together through an OR gate 61. The output of the monostable
multivibrator 46 is correspondingly connected to the second input
of the AND gates 53 and 55 so that these gates are energised only
during the conversion time T.sub.4. The gates 53 and 55 are
therefore associated with the right-hand scanning mark 48 or 48'.
Their outputs are connected to the two inputs of a forward-backward
counter 62, the output of the gate 53 being connected to a forward
input 63 and the output of the gate 55 to a backward input 64 of
the counter 62 which, like the counter 56, is preset to the number
32 and an output 65 of which corresponds to the preselected number
64 while an output 66 is preselected to 0. The function of the
counter 62 corresponds to that of the counter 56. The outputs 65
and 66 are combined through the OR gate 67.
The output of the Schmitt-Trigger 42 which produces the
standardised line starting pulse (see FIG. 4b (c)) is connected to
a pulse reducer 68 which reduces the line starting pulses by a
factor of 8 : 1. After every eighth scanning, a signal appears at
the output of the reducer 68, from which signal the monostable
multivibrator 69 produces a short rectangular pulse of about 40
.mu.s which is transmitted to the two presetting inputs 70 and 71
of the counters 56 and 62. By this arrangement, the counters 56 and
62 are again preset to the number 32 after every eight line
scannings. The operation of the arrangement so far described will
now be explained with the aid of an example. A scanning mark will
first be considered. When the counters 56 and 62 have been preset
to 32 by the arrangement described above, the first of eight line
scannings again begins.
It will be assumed that during the first line scanning the
instantaneous signal value of the noise of the web at the scanning
mark 47 is +0.4 Volt, for example. This value is at the output end
of the sample and hold member 40 during the hold time, is converted
into a pulse sequence in the voltage-frequency converter 50, and is
counted into the forward input 57 of the counter 56 through the
energised AND gate 52 during the conversion time T.sub.2 = 500
.mu.s. At the given conversion ratio of 10 kHz/Volt, this
corresponds to a number of 10.sup.4 .times. 0.4 .times. 5 .times.
10.sup.-.sup.4 = 2 pulses. The counter 56 therefore has the new
value 32+2=34. It will be assumed that in the next, i.e. second
line scanning, the instantaneous signal value at the (left)
scanning mark 47' is -1.2 Volt, for example. This value is
converted into a pulse sequence by the voltage-frequency converter
51 and also put into the backward input 58 of the counter 56
through the energised AND gate 54 during the conversion time
T.sub.2 = 500 .mu.s so that 10.sup.4 .times. 1.2 .times. 5 .times.
10.sup.-.sup.4 = 6 pulses are substracted (counted backwards) from
the value 34 stored there, and the new counting value is now
34-6=28. This process is continued during the next 6 scannings
(three to eight). After that, the counter is preset to the number
32 as described above.
Since as a result of the movement of the web and the traverse
movement of the measuring head the line scannings extend every time
over different regions of the web, the surface noise of the web
represented in FIG. 4b (a) appears to be statistically distributed
so that the process described above forms the digital average value
for eight scannings of the surface noise of the web since neither
the preselected number 64 is reached at the output 59 of the
counter 56 nor the preselected number 0 at the output 60, in other
words no pulse appears at the output of the OR gate 61.
The process is similar for the right-hand scanning mark 48 or 48',
etc. since the arrangement is symmetrical. In this case the sample
and hold member 40, voltage-frequency converter 50, 51, monostable
multivibrator 46, gates 53 and 55, counter 62 and OR gate 67 come
into operation.
In the event of a longitudinal fault, a uniform positive or
negative pulse appears inside the statistically distributed noise
of the web. During the traverse movement of the scanner, first one
scanning mark and then the other passes over this pulse at the
traversing velocity and the pulse is continuously scanned. This is
represented by way of example in FIG. 4c, (a)-(c) which represents
a positive fault impulse 72 and traversing movement of the scanner
from right to left. In this Figure, therefore, the fault pulse 72
moves from left to right in relation to the scanner, i.e. in
relation to the scanning marks 47, 47' and 48, 48'.
FIG. 4c (c) shows the first of the eight scannings which as
described above, make up a scanning cycle takes place. The fault
pulse 72 has just been covered by the scanning mark 47. In the next
following scanning movement, represented in FIG. 4c (b), the
scanning mark 47' covers the fault pulse 72 at another point since
the fault pulse 72 has moved slightly to the right relatively to
the scanning mark due to the traversing movement. This process is
repeated during the following six scanning movements of the cycle
so that the fault pulse 72 is scanned by the scanning mark
altogether eight times, each time at a different point. The time
required for this is 8.T = 8 .times. 1.25 ms = 10 ms. In this
example, therefore, the fault pulse must persist for this length of
time during the passage of the scanning mark. This will be the case
even with narrow pulses since the traversing velocity is very slow,
e.g. 1 mm/10 ms. Since longitudinal fault pulses are uniform
pulses, in this example the fault pulse 72 is always positive, each
scanning is accompanied by voltage frequency conversion in the
voltage frequency converter 50 (FIG. 4a) so that when the first
scanning mark passes over the fault, pulses are counted only
through the forward input 57 of the counter 56 by way of the AND
gate 52 which is energised by the monostable multivibrator 44. The
fault pulse 72 is recognised as a fault if, starting from the
preset counter content 32, a further 32 pulses have been counted in
after eight scanning movements so that the preselected number 64 is
reached and consequently a pulse appears at the output 59 of the
counter 56 and therefore at the output of the OR gate 61. This
happens when the average scanned pulse value during the eight
scannings of the fault pulse 72 is just equal to +0.8 Volt because
the number of pulses counted into the counters 56 when the
conversion ratio of the voltage frequency converter 50 is 10
kHz/Volt, the conversion time T.sub.2 = 500 .mu.s and eight
scanning movements are carried out is then 10.sup.4 .times. 0.8
.times. 500 .times. 10.sup.-.sup.6 .times. 8 = 32. It can easily be
checked that even the ascending flank of a triangular pulse lasting
20 ms and having an amplitude of 1.6 Volt corresponds to an average
scanned pulse value of 0.8 Volt for eight scannings.
The amplitude of the fault pulse 72 may well only be equal to or
even be smaller than the amplitude values of the noise of the web
since the noise has no influence on the recognition of a fault
because its amplitudes are averaged out.
After another traverse movement, the fault pulse 72 reaches the
second scanning mark 48 (FIG. 4c (c)) which also recognises it as a
fault (by the same procedure) as already described above, and in
this case (FIG. 4a ) the sample and a hold member 40, the voltage
frequency converter 50, the monostable multivibrator 46, the AND
gate 53, the counter 62 and the OR gate 67 come into operation in a
corresponding manner.
The recognition of negative longitudinal fault pulses takes place
in an analogous manner. In this case, the voltage frequency
converter 51 and the AND gates 54 and 55 which are connected to the
backward counting inputs 58 and 64 of the counters 56 and 62, in
which the numbers are substracted from the preset number 32, come
into operation. When the preselected number 0 is reached, a pulse
appears at the outputs 60 (for the left scanning mark) or 66 (for
the right-hand scanning mark)) and hence at the outputs of the OR
gates 61 and 67.
To ensure that only longitudinal faults and no point faults are
recognised, a given fault must be detected by both scanning marks
during the traversing movement. The logical decision that the marks
are dealing with one and the same fault is made in the circuit
connected to the outputs of the OR gates 61 and 67. That circuit
consists of two monostable multivibrators 73 and 74, AND gates 75
and 76 and an OR gate 77. The monostable multivibrators contain
control inputs 78 and 79 to which a positive control voltage must
be applied to release the multivibrator.
So long as such a voltage is not supplied, the multivibrator is
blocked. This control voltage is tapped from the motor control 34
(FIG. 3) provided for the traverse movement, that is to say for a
traverse movement from right to left a control voltage is supplied
only to the control input 78 so that the monostable multivibrator
73 is released while the monostable multivibrator 74 and hence also
the gate 76 remain blocked. When the traverse movement is from left
to right, the multivibrator 74 is released while the multivibrator
73 and hence the gate 75 remain blocked.
When the scanner traverses from right to left as in the previous
example, a pulse appears at the output of the OR gate 61 if the
left scanning mark recognises the fault. This pulse releases the
monostable multivibrator 73 which therefore produces a pulse which
keeps the AND gate 75 open until the right-hand scanning mark
recognises the fault so that the pulse appearing at the output of
the OR gate 67 is transmitted to the recording or storage device 80
by way of the open AND gate 75 and OR gate 77. This recording or
storage device 80 may also contain some function associated with
the width of the faulty web derived from the transmitter 38 (FIG.
3).
When the scanner traverses from left to right, recognition of the
fault at the right-hand scanning mark energises the monostable
multivibrator 74 since a pulse appears at the output of the OR gate
67. The pulse of the multivibrator 74 then keeps the AND gate 76
open until the left-hand scanning mark detects the fault so that
the pulse appearing at the output of the OR gate 61 reaches the
processing unit 80 by way of the open AND gate 76 and the OR gate
77.
The length of the pulses produced by the two multivibrators 73, 74
must be greater than the time taken for the fault to move
relatively from one scanning mark to the other during the traverse
movement. At a traversing velocity of 1m/10 ms, for example, and a
distance between the scanning marks of 40 mm, this time is 400 ms
and consequently it is sufficient for the multivibrator pulse to
last 500 ms.
The minimum length at which a fault will still be recognised as
longitudinal fault depends on the distance between the two scanning
marks, the traversing velocity and the velocity of longitudinal
movement of the web of material. Starting from a time of 400 ms for
the relative movement of the fault from one scanning mark to the
other, this length of fault is 400 mm if the web moves
longitudinally at the rate of 60 m/min = 1 mm/ms, for example.
Two point faults could simulate a longitudinal fault pulse if one
of the faults is recognised by one scanning mark and the other
fault by the other but the likelihood of such a combination of
faults is very slight, particularly since a longitudinal fault
could be simulated only if this coincidence were repeated several
times.
The method described above for recognising longitudinal faults also
enables faults close to the edge of the web of material to be
detected. This is illustrated diagrammatically in FIG. 5 for the
left-hand edge of the web of material 8. The traversing movement is
from right to left (FIG. 5a). The longitudinal fault is first
recognised by the left scanning mark 47. Shortly before the mark
reaches the left-hand edge of the web, the AND gates 52 and 54
which are associated with the left scanning mark are blocked by
their common input 82 which is otherwise open (FIG. 4a). This
blocking of the AND gates is due to a signal which is produced by
the detecting element 38 for the width of the web (FIG. 3) and
which places the input 82 at zero potential. The left-hand scanning
mark 47 is then unable to recognise the edge of the web as a fault.
When the right-hand scanning mark 48 has also recognised the fault
81, the traverse movement from right to left is terminated by the
limit switch 35 (FIG. 3) at the moment when the scanning mark 48
has just reached the left-hand edge of the web but is still on the
web (FIG. 5b). When the traverse movement from left to right then
begins, the blocked gates 52 and 54 are released by the zero
potential at the input 82 being switched off, but only when the
left-hand scanning mark is again situated on the web 8, so that the
fault 81 is unambiguously detected, first by the right-hand
scanning mark 48 and then by the left-hand scanning mark 47.
Recognition of a fault at the right-hand edge of the web takes
place in an analogous manner, the right-hand scanning mark 48 being
in this case rendered inoperative by blockage of the AND gates 53
and 54 by way of their common input 83 shortly before the scanning
mark leaves the web. When the traverse movement is reversed, the
blocking is removed as soon as the scanning mark 48 is again on the
web 8.
The figures given to illustrate the example more clearly are purely
arbitrary. They can be replaced by any other figures adapted to the
problem.
In order to form the average value described above, it is
advantageous always to start with approximately the same amplitude
values of the web surface noise regardless of the particular web of
material being examined so that once the values for amplification,
counter presetting, counter preselection, etc. have been adjusted,
they need not be reset each time the apparatus is used to
investigate a new material or a material in which the surface
properties alter in the course of the examination. For this
purpose, the intensity of the scanning lamp 1 (FIG. 1) is
automatically adjusted so that the surface noise signal transmitted
to the photoelectric receiver 13 (FIG. 1) remains constant. This is
illustrated in FIGS. 6a and 6b.
FIG. 6a shows the circuit diagram of the control circuit for the
lamp intensity, 6b is the corresponding pulse-time diagram for two
successive scannings. The control circuit proper (FIG. 6a ) is
connected between the points 84 and 85 of FIG. 4a. For the sake of
completeness and clarity, the photoelectric receivers 13 and 16,
amplifiers 39 and 41 and Schmitt-Trigger 42 have been shown again
in FIG. 6a. In the pulse diagram FIG. 6b, the diagrams a, b and c
are identical with diagrams a, b and c of FIG. 4b. FIG. 6b (a)
therefore shows the output signal of the photoelectric receiver 13
amplified in the amplifier 39, comprising the incoming pulse 17 and
outgoing pulse 18 or the pulses 17' and 18' for the following
scanning. FIG. 6b (b) shows the variation with time of the line
starting pulse 19 or 19' for the following scanning and FIG. 6b and
(c) shows the standardised rectangular pulse obtained therefrom at
the output of the Schmitt-Trigger 42.
The monostable multivibrator 86 is connected to the output of the
Schmitt-Trigger 42. Its own output is connected to the gate of a
field effect transistor 87 (self-blocking n-channel FET). Triggered
by the negative rear flank of the Schmitt-Trigger pulse (FIG. 6b
(c)), the monostable multi-vibrator 86 delivers a positive
rectangular pulse (FIG. 6b (d)). For the duration of this pulse,
the field effect transistor 87 which is normally blocked becomes
energised. The pulse duration is selected so that only the incoming
pulse 17 (or 17'), whose amplitude is a measure of the intensity of
the light beam reflected by the web of material, can pass through
the field effect transistor 87. FIG. 6b (e) shows the corresponding
pulse diagram at the output of the field effect transistor 87. The
incoming pulses pass from the output of the field effect transistor
87 to a peak value detector 88 e.g. (Type APM 1, Loetscher
Elektronik) the output of which is connected to an input 92 (actual
value input) of a difference amplifier 93 by way of the voltage
divider which consists of the resistances 89 and 90 and a
capacitance 91 which is connected in parallel with the resistance
90. A voltage transmitter 94 feeds a fixed positive direct voltage
as a reference value to a second input 95 (reference value input)
of the difference amplifier 93. The output of the difference
amplifier 93 is connected to the base of an npn output control
transistor 96 which is connected in series with the scanning lamp 1
and which has the lamp supply voltage fed into it at the
collector.
The peak value detector 88 has the function of recognising the
maximum of the incoming pulse 17 or 17' for each scanning and
storing it analogously. The loading time of the peak value detector
should be short compared with the scanning time T while the
unloading time is such that the stored peak value can drop by about
1 percent per scanning so that a direct voltage corresponding to
the peak value is available at the output of the peak value
detector 88 as a measure of the intensity of the beam of light
reflected by the web. The output signal of the peak value detector
88 is represented in FIG. 6b (f).
Additional smoothing of this signal is effected by the RC member
89, 91. Where a transition from higher to lower voltage values
occurs due to the high resistance of the amplifier input 92, the
resistance 90 serves for more rapid discharge of the condenser 91,
but R.sub.1 C must be .ltoreq.R.sub.2 C (e.g. R.sub.1 C=1s, R.sub.2
C=10 s).
The voltage at the actual value input 92 of the difference
amplifier 93 is therefore a positive direct voltage corresponding
to the intensity of the beam reflected by the web.
The control operation proceeds as follows. If the reflection
properties of the web of material under investigation are reduced,
for example due to a change of web, so that the intensity of the
reflected beam and hence of the incoming pulse 17 are reduced, then
the actual value direct voltage at the input 92 of the difference
amplifier 93 also drops and consequently the difference between the
fixed reference value voltage at the input 95 and the actual value
voltage at the input 92 is increased. The output of the difference
amplifier 93 therefore becomes more positive so that the npn output
control transistor 96 is further energised and the intensity of the
lamp 1 is increased. This process continues until the actual value
reaches the reference value and hence the intensity of the
reflected beam again reaches the original value.
Control in the reverse direction from higher to lower intensity
takes place in an analogous manner.
To prevent the intensity control being affected by large
interfering impulses such as may occur, for example, where parts of
the web are glued, a Zener diode 98 is connected to the output of
the amplifier 39 as an impulse cutting stage for large pulses. The
Zener voltage is sufficiently high for the control not normally to
be affected.
This automatically intensity control ensures that the intensity of
the beam reflected by the web remains constant regardless of the
reflection properties of the material under investigation so that
the values which must be preadjusted for detecting a particular
longitudinal fault, such as the amplification, presetting of the
counter, preselection of the counter, etc. need only be adjusted
once.
* * * * *